The present invention relates to an optical information reproducing apparatus for reproducing a next-generation high density optical disc, as well as to an optical head and an objective lens optical system which are incorporated therein.
Recent years, high-density recording of an optical disc has been steadily developed, and in a digital versatile disc (DVD) the storage capacity of both of a read-only memory disc (ROM) and a rewritable disc (RAM) is as high as 4.7 GB. In addition to this, in recent years at which satellite broadcasting is to be digitized immediately, the optical disc is expected to be large capacity of 20 GB or more where high definition moving picture can be recorded for two hours or more.
A size of a light beam spot that directly restricts a recording density of the optical disc is given as xcex/NA when a wavelength of a light beam is represented as xcex and a numerical aperture of an objective lens is represented as NA. Accordingly, the wavelength must be set short or the numerical aperture must be set large in order to realize an optical disc with large capacity. With respect to the wavelength, development of a blue-violet laser diode which emit a light beam of 405 nm has been advanced, and it has been forecasted to realize an optical disc with capacity of 12 GB that is about 2.6 times as large as the present DVD for which a light beam of 650 nm is used. In order to further increase the capacity to 20 GB or more, NA must be increased to be 1.3 times as large as the present DVD, that is, 0.77 or more.
As the conventional technology to increase NA as described above, there has been, for example, Japanese Patent Laid-Open No. 11(1999)-195229. In this technology, NA is increased to 0.85 as the maximum value by use of an objective lens which is composed of two lens elements in two groups. Accompanied with the increase of NA at this time, there is a problem that a spherical aberration and a coma, which are caused by an error of a substrate thickness of a disc and an inclination thereof, increase. To cope with such a problem, the coma due to the inclination of the disc is decreased by decreasing the substrate thickness to 0.1 mm, and the spherical aberration due to the error of the substrate thickness is compensated by detecting the substrate thickness based on a difference of a focus error signal between a disc surface and a recording surface and by changing an interval between the two lenses in response to the substrate thickness. Herein, an interval between a final surface of the lens system and the surface of the substrate surface of the disc when a recording film is focused, that is, a working distance, is 0.13 mm, and an effective pupil diameter of the two-element objective lens is 3 mm xcfx86.
In the above described prior arts, when the working distance is very short and a focus servo comes off by any chance during a recording/reproduction operation, the lens collides against the disc and the disc may be damaged. Moreover, there is a problem that a permissible limit of the interval between the two lenses and decentering of the two lenses relative to each other are very strict and adjustment is difficult.
The simplest and essential means for widen the working distance of the high NA lens in the above described prior arts is to use one lens in stead of the two lenses. This means is described by use of FIGS. 1(a) and 1(b). FIG. 1(a) shows a state where the conventional two lenses 101 and 102 collect a light beam, and FIG. 1(b) is a schematic view showing a difference of the working distance when the light beam collection is performed by one high NA lens 103. In both of FIGS. 1(a) and 1(b), luminous flux is collected in a recording film 105 through a protection layer 104 of a disc by the same NA. In FIG. 1(a), in order to respectively distribute refractive power to the two lenses 101 and 102, the second lens 102 will be inserted in the luminous flux collected by the first lens 101. Therefore, compared to the case of FIG. 1(b) where the luminous flux is collected by one lens, an effective pupil diameter D2 of the lens is smaller than an effective pupil diameter D of FIG. 1(b), and a working distance WD1 is obliged to be shorter than a working distance WD2 of FIG. 1(b). On the contrary, the working distance can be further widened by collecting the luminous flux by one lens than by collecting the luminous flux by the conventional two lenses. However, as a matter of course, the reason why the two lenses are necessary has heretofore been existed. This is a problem in manufacturing the lens. In a lens offering a large NA, necessary precision concerning a decentering between both surfaces of the lens and an error of a surface interval between both surfaces of the lens is very strict, and large aberration occurs by a slight error. To avoid the aberration, by dispersing the necessary refractive power conventionally to the two lenses, manufacturing of the respective lenses is made easier. Accordingly, to acquire a high NA with one lens, either a manufacture technology to increase a positional precision between both surfaces of the lens or means for compensating the aberration caused by a manufacture error is necessary.
With respect to means for compensating the aberration, there is, for example, means described in Japanese Patent Laid-Open No. 12(2000)-182254, which compensates a spherical aberration caused by the error of the surface interval. In this Japanese Patent Laid-Open No. 12(2000)-182254, a spherical aberration at a light convergence spot is optically detected, and luminous flux made incident on an objective lens is slightly diverged or converged, whereby a spherical aberration compensation optical system for causing spherical aberration is driven and spherical aberration of an optical system is compensated. The error of the surface interval between the first and second surfaces of one lens with a high NA can be compensated by combining such technologies. At this time, the light beam itself made incident on the lens does not show aberration, and spherical aberration occurs in an objective lens by allowing the light beam, which changes its diversion/conversion state, to be incident on the objective lens. Accordingly, even if the lens moves from an optical axis in a radius direction of a disc by a tracking operation, an axis of the spherical aberration generated does not swerve substantially.
With respect to coma caused by decentering, for example, a detection method of coma is described in Japanese Patent Laid-Open No. 12(2000)-214048. In this Japanese Patent Laid-Open No. 12(2000)-214048, the coma in a radius direction of a disc is detected based on difference of a push-pull signal between an inner side of luminous flux and an outer side thereof, and coma in a tangent direction is detected based on difference of a push-pull signal in a diagonal direction among four segmented regions split in a radius direction of the disc and a tangent direction thereof. As compensation means, for example, in Japanese Patent Laid-Open No. 13(2001)-4972, a technology is described, in which coma is given by giving a phase distribution having a sign reverse to that a W-shaped phase distribution given in the form of a fourth order function with a shift. By combining these technologies, the detection of the coma and the compensation thereof are made possible. However, in this case, the coma does not occur in the objective lens unlike the case of the foregoing spherical aberration compensation, and the coma occurs in a compensation device of the coma. Accordingly, if the objective lens deviates from an optical axis by a tracking operation, an axis of the coma deviates in accordance with the deviation of the objective lens. In this case, since the axis of the coma of the objective lens to be compensated and the axis of the coma compensated swerve from each other, astigmatism occurs substantially owing to a difference of the axes. This can be understood by the following equations simply. When coordinates in a section of luminous flux is expressed by polar coordinates (xcfx81, xcex8) while using the optical axis as the origin, the coma is given by the equation (1),
W1=W31xcfx813 cos xcex8xe2x80x83xe2x80x83(1)
where W31 is a Seidel""s aberration coefficient representing a scale of the coma, and a direction where the coma occurs is assumed to be a 0xc2x0 direction. This is assumed to be a coma of the objective lens to be compensated. Next, when it is assumed that the lens swerves to the 0xc2x0 direction by xcex94, a coma substantially compensated by a coma corrector is given by the equation (2),                                                                                           W                  2                                =                                ⁢                                                      W                    31                                    ⁢                                                            {                                                                                                    (                                                          x                              -                              Δ                                                        )                                                    2                                                +                                                  y                          2                                                                    }                                                              3                      /                      2                                                        ⁢                  cos                  ⁢                                      xe2x80x83                                    ⁢                  θ                                                                                                        ≅                                ⁢                                                                            W                      31                                        ⁢                                          ρ                      3                                        ⁢                    cos                    ⁢                                          xe2x80x83                                        ⁢                    θ                                    -                                      3                    ⁢                                          W                      31                                        ⁢                                          Δρ                      2                                        ⁢                                          cos                      2                                        ⁢                                          xe2x80x83                                        ⁢                    θ                                                                                      "AutoLeftMatch"                            (        2        )            
where (x, y) represents orthogonal coordinates and is related to the polar coordinates by the following equation (3).
xcfx812=x2+y2xe2x80x83xe2x80x83(3)
Accordingly, it is proved that the aberration after compensation is a function which takes the form of astigmatism in proportion to the swerve xcex94 as shown by the equation (4).
W1xe2x88x92W2≅3W31xcex94xcfx812cos2 xcex8xe2x80x83xe2x80x83(4)
From the viewpoint of the above described problems, in compensating coma of the aberration compensation optical system which realizes one objective lens with a wide working distance and a high NA and which can be adjusted easily, an object of the present invention is to reduce astigmatism caused by swerve of an axis of coma of the aberration compensation optical system compensated from an axis of coma thereof to be compensated accompanied with a tracking operation.
To solve the foregoing subject, in the present invention, used is an objective lens optical system in which a coma corrector and one objective lens formed of a single medium are united with each other through a case so that both are positioned fixedly while keeping a mutual positional relationship therebetween, the one objective lens having a numerical aperture of 0.8 or more and collecting luminous flux incident thereon through the coma corrector beyond a transparent substrate.
In an optical head, in a light beam convergence optical system which collects a diverging light beam from a semiconductor laser in the optical head onto an optical information recording medium having a transparent protection layer, the foregoing objective lens optical system is loaded on an actuator which makes a position of a light convergence spot onto the optical information recording medium variable, and a reflection light beam from the optical information recording medium is split from an optical path extending from the foregoing semiconductor laser to the optical information recording medium by a beam splitting device to be guided to a photodetector by a photo detection system, followed by detection of an intensity of the reflection light beam as an electric signal. Herein, the optical head is constituted by parts from which a light beam emits and onto which a light beam is incident, and by parts such as a holder and a case, which directly hold the parts except for an optical disc. The optical head generally does not comprise an arithmetic circuit for a signal output from the photodetector and a driving circuit for driving a light beam source.
In an optical information reproducing apparatus using such an optical head, a light convergence spot position control signal detection circuit is constituted based on the output signal from the photodetector together with the optical information reproducing apparatus, and the actuator is driven by a control signal.
Furthermore, as one of other optical information reproducing apparatus according to the present invention, two actuators of a fine actuator and a coarse actuator are used as the actuator which makes the light convergence spot on the optical information medium variable. The fine actuator drives an objective lens with a NA of 0.8 or more independently from a coma corrector, and compensates high-frequency components of a tracking signal. The coarse actuator drives a portion including a light beam convergence optical system and the fine actuator, or the whole of the optical head including these to compensate low-frequency components of a tracking error signal. In the case where the whole of the optical head is loaded on the coarse actuator, in a light beam convergence optical system of the optical head which collects a diverging light beam from the semiconductor laser onto the optical information recording medium having a transparent protection layer, in a case accommodated unitedly are a beam splitting device for splitting a reflection light beam from an optical information recording medium from an optical path extending from a semiconductor laser to an optical information recording medium; a detection optical system for guiding the reflection light beam split from the beam splitting device to a photodetector; and the photodetector for detecting an intensity of the reflection light beam as an electric signal. In the case where the portion including the light beam convergence optical system and the fine actuator is loaded on the coarse actuator, the optical head is divided into a fixed optical system including the semiconductor laser and the photodetector and a movable optical system loaded on the fine actuator.
In the above described optical head and optical information reproducing apparatus, it is possible to compensate spherical aberration resulting from spherical aberration indigenous to a lens and a thickness error of a disc substrate by loading a spherical aberration compensator on the light beam convergence optical system.
In the above described optical head and optical information reproducing apparatus, both surfaces of the coma corrector may be formed by a rotation symmetry fourth order curved surface with a curvature on the center axis equal to approximately zero. Alternatively, the coma corrector may be a liquid crystal phase compensator.
It is possible to provide the optical disc apparatus with a high reliability by the present invention, in which the objective lens having a large NA and a working distance can be realized by the present invention, and in which a lens hardly collides with the disc.